Lithium disilicate glass-ceramic, method for production thereof and use thereof

ABSTRACT

The invention relates to glass-ceramics based on the lithium silicate system which can be mechanically machined easily in an intermediate step of crystallisation and, after complete crystallisation, represent a very strong, highly-translucent and chemically-stable glass-ceramic. Likewise, the invention relates to a method for the production of these glass-ceramics. The glass-ceramics according to the invention are used as dental material.

This application claims priority to and is a continuation of U.S.application Ser. No. 14/274,235 filed May 9, 2014, which published as USPublication No. 2014/0249016 on Sep. 4, 2014, now U.S. Pat. No.10,219,879, which is a continuation of U.S. application Ser. No.13/518,765 filed Aug. 15, 2012, which published as U.S. Publication No.2012/0309607, now U.S. Pat. No. 8,956,987, which is the national stageentry of PCT/EP10/07918 filed Dec. 23, 2010, which claims priority to DE10 2009 060 274.4 filed Dec. 23, 2009, all of which are herebyincorporated by reference in their entirety.

The invention relates to glass-ceramics based on the lithium disilicatesystem which can be mechanically machined easily in an intermediate stepof crystallisation and, after complete crystallisation, represent a verystrong, highly-translucent and chemically-stable glass-ceramic.Likewise, the invention relates to a method for the production of theseglass-ceramics. The glass-ceramics according to the invention are usedas dental material.

Lithium disilicate glass-ceramics are well known from the literature andseveral patents are based on this glass-ceramic system. Thus, forexample, self-glazed lithium disilicate glass-ceramic objects for theproduction of tableware are described in EP-B-536 479, in EP-B-536 572lithium disilicate glass-ceramics which can be used, by scatteringfine-particle coloured glass on the surface thereof, as lining elementsfor building purposes.

The main focus of patented lithium disilicate glass-ceramics is ondental applications. This is due to the fact that the crystallisation oflithium disilicate crystals is effected via a phase of lesser strength(lithium metasilicate) and the material system is consequently amenableinter alia to chair-side methods (see S. D. Stookey: “Chemical Machiningof Photosensitive Glass”, Ind. Eng. Chem., 45, pp. 115-118 (1993) and S.D. Stookey: “Photosensitively Opacifiable Glass” U.S. Pat. No. 2,684,911(1954)). Investigations by Borom, e.g. M.-P Borom, A. M. Turkalo, R. H.Doremus: “Strength and Microstructure in Lithium DisilicateGlass-Ceramics”, J. Am. Ceream Soc., 58, No. 9-10, pp. 385-391 (1975)and M.-P. Borom, A. M. Turkalo, R. H. Doremus: “Verfahren zum Herstellenvon Glaskeramiken” (Method for the production of glass-ceramics),DE-A-24 51 121 (1974) show that glass-ceramics which comprise lithiummetasilicate as main phase have reduced strength in comparison withglass-ceramics which comprise lithium disilicate as single crystallinephase.

This principle was used in order firstly to produce a glass-ceramic, ina two-step crystallisation process, which can be machined wellmechanically, e.g. by means of CAD/CAM methods, and to process thissubsequently in a second crystallisation step to form dentalglass-ceramic. This method is suitable for being able to use dentalrestorations according to the so-called chair-side method. In the caseof this method, an individually adapted crown/onlay/inlay is milled outof a glass-ceramic block after the first crystallisation step by meansof CAD/CAM in the dentist's surgery, this is subjected to the secondcrystallisation step in a special oven and used directly in the firstand only dental appointment for the patient (DE 10 2005 028 637).

In addition, in WO-A-95/32678 and U.S. Pat. No. 5,507,981, lithiumdisilicate glass-ceramics were described, which can be processed to formshaped dental products by means of hot-pressing by using a specialcompressible crucible. Furthermore, there are known, from DE-C-14 21886, glass-ceramics based on SiO₂ and Li₂O which contain largequantities of physiologically very questionable arsenic oxide. Also inU.S. Pat. No. 4,515,634 and in FR-A-2 655 264, lithium disilicateglass-ceramics which are suitable for the production of dental crownsand bridges are disclosed.

All known lithium disilicate glass-ceramics display inadequacies in theprocessing thereof to shaped products and/or in mechanical or visualproperties and/or in chemical stability. In particular when used in thedental field, equally high requirements for all the mentioned propertiesmust be fulfilled.

Starting herefrom, it was the object of the present invention to providea glass-ceramic which has improved mechanical and optical properties andalso improved chemical stability relative to the (glass-) ceramics knownfrom the state of the art.

This object is achieved by the lithium disilicate glass-ceramic havingthe features of at least 10% by weight of a stabilizer in order toincrease the chemical and mechanical stability, the stabilizer beginpresent essentially in the amorphous phase and also by the method forthe production of this glass-ceramic having the features in which a) aninitial glass is produced which comprises the components of theglass-ceramic, b) the initial glass is subjected to a first heattreatment in order to produce a glass-ceramic which has lithiummetasilicate as main crystal phase, c) the glass-ceramic of b) issubjected to a second heat treatment in which the lithium metasilicateis converted with SiO₂ from the glass phase into lithium disilicate andsubsequently lithium disilicate is present as main crystal phase. Usesaccording to the invention as dental material or as a component of adental material are indicated. Likewise, a shaped dental product havingthe features of an inlay, an onlay, a bridge, a pint construction, aveneer, and a (partial) crown is provided.

Within the scope of the present invention, glass compositions have beendeveloped which can be prepared in a two-step production process, areeasy to machine after the first crystallisation step, in particular bymeans of CAD/CAM, and, after a very short second crystallisation step,are both highly-transparent and very strong and have better chemicalstabilities than the known lithium disilicate glass-ceramics.

It was shown surprisingly that the addition of at least 10 wt. % ZrO₂ tocertain glass compositions leads to glass-ceramics which can be machinedvery readily in an intermediate crystallisation step and, in the endstate, have excellent strength values, exceptional translucence andsignificantly increased chemical stabilities.

It was shown that up to 20% by weight of a stabiliser selected from thegroup consisting of ZrO₂, HfO₂ or mixtures hereof can be incorporated inthe glass without having a significant influence on the structure.Contrary to all expectations, the stabiliser does not hereby crystalliseout as a separate crystal phase but remains in the remaining glassphase. As a result of the high proportion in the amorphous phase, themechanical and chemical stabilities in this phase are hugely improved,which also leads to improved properties in the end product.

In particular the chemical stability can be improved via the compositionof the remaining glass phase since the glass phase has a significantlyhigher solubility than the lithium disilicate and hence represents theweak point with respect to chemical attack. The extremely highsolubility of the stabiliser (ZrO₂) in the glass phase is in particularremarkable since e.g. zirconium oxide acts in many silicateglass-ceramics as nucleation agent, i.e. crystallises out as first phaseduring a temperature treatment, and the actually sought crystal phase isfacilitated and is deposited in a fine-crystalline manner on these ZrO₂crystals.

As a result of the high proportions of stabiliser which remainessentially in the amorphous phase, the crystalline proportion iscorrespondingly restricted. As a result, and due to the low crystallitesize of the lithium disilicate crystals, good translucence of thematerials is produced after the second crystallisation. The translucenceis however also further improved by the refractive index of the glassphase being increased in turn by the stabiliser and, consequently, beingadapted to the refractive index of the lithium disilicate. In the caseof glass-ceramics in which the refractive index of the amorphous matrixphase corresponds to the refractive index of the crystallinephase/phases, very good translucence properties are found, relativelyirrespective of the crystallite size. In the glass-ceramics according tothe invention, therefore all three points for the production of anextremely translucent glass-ceramic are fulfilled:

-   -   limited crystal phase proportion,    -   small crystals (<500 nm),    -   adapted refractive index of amorphous and crystalline phase.

The high proportion of stabiliser has the effect therefore in theglass-ceramic of improved chemical stability, higher strength values andimproved translucence in several respects to correspondingglass-ceramics without or with only a low ZrO₂— or HfO₂ proportion.

The glass-ceramics according to the invention can be produced preferablyby means of a method, in which

a) an initial glass is produced which comprises the components of theglass-ceramic,

b) the initial glass is subjected to a first heat treatment at a firsttemperature in order to produce a glass-ceramic which has lithiummetasilicate as single or main crystal phase and

c) this glass-ceramic is subjected to a second heat treatment in whichthe lithium metasilicate is converted with SiO₂ from the glass phaseinto lithium disilicate and subsequently lithium disilicate is presentas single or main crystal phase.

The crystallisation to form lithium metasilicate preferably takes placeat temperatures between 620° C. and 800° C., with times between 1 and200 minutes, preferably between 650° C. and 750° C. for 10 to 60minutes.

The crystallisation to form lithium disilicate preferably takes place attemperatures between 800° C. and 1,040° C., with times of 5 to 200minutes, preferably between 800° C. and 870° C. for 5 to 30 minutes.

The subject according to the invention is intended to be explained inmore detail with reference to the subsequent examples without wishing torestrict said subject to the special embodiments shown here.

EXAMPLES 1 TO 6

In examples 1 to 6, compositions of glasses with a high zirconium oxidecontent are indicated, which are converted by a two-step temperaturetreatment firstly into readily mechanically machinable lithiummetasilicate glass-ceramics and subsequently into highly-translucent,very strong and chemically-stable lithium disilicate glass-ceramics.

The compositions with their components are represented in Table 1.

TABLE 1 81 82 83 84 85 86 Si0₂ 66.9 65.8 65.5 63.7 63.5 63.5 Li₂0 13.913.7 13.6 13.2 14.4 12.9 Zr0₂ 10.0 10.0 12.0 11.7 12.7 13.5 Al₂0₃ 3.23.1 3.1 3.0 3.3 3.5 P₂0₅ 3.0 3.0 3.0 2.9 3.1 3.4 K₂0 2.9 2.9 2.9 2.8 3.03.2 Ce0₂ — 1.0 — 2.0 — — Er₂0₃ — 0.2 — 0.3 — — Tb₂0₃ — 0.3 — 0.3 — —

The glasses were melted at 1,500° C. and poured into metal moulds toform blocks. The blocks were stress-relieved at 560° C. in the furnaceand cooled slowly. For the different characterisation processes, theglass blocks were divided up and subjected to a first crystallisationtreatment. For this purpose, the glasses were aged for 10 to 120 minutesat 600° C. to 750° C. As a result, glass-ceramics with strength valuesof 150 MPa to 220 MPa were produced. Exclusively lithium metasilicatewas hereby established as crystal phase. In this state, machining bymeans of CAD/CAM methods is very readily possible.

With a second short crystallisation at 800° C. to 950° C. for 3 to 15minutes, recrystallisation of the lithium metasilicate with amorphousSiO₂ from the glass phase takes place to form lithium disilicate and theresult is an increase in strength to 300 MPa to 450 MPa. In addition tothe lithium disilicate phase, a subsidiary crystal phase with azirconium oxide content can hereby be produced. In addition, also smallresidues of lithium metasilicate can be present. The unequivocal maincrystal phase is lithium disilicate.

In Table 2, the crystallisation conditions of individual glasses andalso the resulting crystal phases and strength values are displayed.

TABLE 2 Glass 81 82 83 84 85 86 1. Crystallisation 650° C. 700° C. 650°C. 700° C. 700° C. 700° C. 20 min 40 min 30 min 20 min 40 min 40 min 2.Crystallisation 850° C. 830° C. 870° C. 850° C. 820° C. 830° C. 10 min10 min 20 min 8 min 10 min 10 min Crystal phases Main phase disilicatedisilicate disilicate disilicate disilicate disilicate (>80%) Subsidiaryphase — — — — metasilicate metasilicate (<20%) Translucence excellentvery good excellent very good excellent excellent 3-point 375 MPa 413MPa 380 MPa 418 MPa 356 MPa 385 MPa bending strength

The invention claimed is:
 1. A lithium silicate glass ceramic thatincludes at least 10 wt % ZrO₂, from 55 to 70 wt % SiO₂, and from 10 to15 wt % LiO₂ and having a main crystal phase consisting of lithiumdisilicate crystal.
 2. Lithium silicate glass ceramic of claim 1, whichis in the form of a dental restoration.